1. Field of the Invention
The present invention relates to a direct fuel-injection engine having a cavity recessed in a central part of the top face of a piston, and a fuel injector for injecting fuel into the cavity. The fuel injector carries out main injection along a fuel-injection axis oriented toward an inner wall face of the cavity when the piston is in the vicinity of top dead center, and carries out secondary injection along the same fuel-injection axis when the piston is positioned further from top dead center than during main injection.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 7-324661 (JP '661) discloses a direct fuel-injection diesel engine in which a fuel injector is provided with a first injection hole for pilot injection and a second injection hole for main injection. JP '661 teaches that when fuel is subjected to pilot injection (secondary injection) from the fuel injector before a piston reaches top dead center, and fuel is subjected to main injection from the fuel injector when the piston has reached top dead center, fuel is made to collide at an appropriate position within a cavity by setting a small cone angle for fuel that is subjected to pilot injection and a large cone angle for fuel that is subjected to main injection.
Furthermore, Japanese Patent Application Laid-open No. 2001-227345 (JP '345) discloses a direct fuel-injection diesel engine that carries out main and secondary injections along the same fuel-injection axis, wherein the direction of the fuel-injection axis when carrying out secondary injection is oriented toward the open end of a cavity of a piston.
Incidentally, the arrangement disclosed by JP '661 requires the structural arrangement of the fuel injector to be rather complex since it is necessary to provide the fuel injector with the first injection hole for pilot injection and the second injection hole for main injection, thus causing an increase in manufacturing cost.
Furthermore, the arrangement disclosed by JP '345 poses a problem wherein unused air could possibly remain in an upper part of the cavity since the direction of the fuel-injection axis when carrying out secondary injection is oriented toward a high position (open end) of the cavity, the direction of the fuel-injection axis when carrying out main injection is also oriented toward a relatively high position of the cavity, and fuel that has collided with an inner wall face of the cavity, which enlarges in a downward facing direction, is not directed to the open end side of the cavity but is directed to a bottom wall side.